Solid, beefy ans stable
They are used in my Soldano HR50. Before, i had both Gold Lion and Shungang. While those might not give you the most authentic KT66 tone, they sound very good, giving a lot of big fat power. And they last forever. Might not be for those who like power tube distortion, but if your amp relies on preamp overdrive, such as the Soldano, well, they just complement the amp's tone perfectly. In short: lots of headroom, plenty of power, very sturdy and stable and, last, a guitar-friendly frequency curve.